Preparation of thin film superconducting oxides

ABSTRACT

Cations such as La, Sr, Cu, or Y, Ba, Cu are dissolved in an organic solvent such as ethylene glycol and citric acid. The solution is formed into either a free-standing or supported film which is dried to produce a solid organic polymer. The polymer is then fired in an oxidizing atmosphere (pyrolysis) to obtain the superconducting oxide. It is preferred that the film be spin coated on a substrate to produce uniform coatings of thicknesses less than one micrometer. The resulting superconducting oxide film is fully dense, of controlled microstructure, very monogeneous in composition and suitable for demanding electronic device purposes or as coatings to form superconducting wires or other current carrying components.

BACKGROUND OF THE INVENTION

This invention relates to the preparation of thin superconducting oxidefilms.

Superconductors are materials having substantially zero resistance tothe flow of electrons below a certain critical temperature. It is knownthat certain metal oxides exhibit high temperature superconductivity,that is, critical temperatures greater than 30K. Included are La_(2-x)M_(x) CuO_(4-y), where M is an alkaline earth cation (e.g., Ba, Sr, Ca)and in which the critical temperature can be greater than 35 K, and NBa₂Cu₃ O_(7-x), where N can be Y, La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu,etc. and in which the critical temperature can be greater than 90K. Itis desirable to provide such oxides in a thin film or coating which isfully dense, of controlled microstructure, and very homogeneous incomposition, for demanding electronic device purposes such asstriplines, Josephson junction devices, superconducting quantuminterference devices (SQUID) and as superconducting coatings on asupport material such as a wire, sheet or coil in order to make asuperconducting component.

SUMMARY OF THE INVENTION

According to the invention, an organic liquid solution containing thecation constituents of the oxide film is made. This liquid contains anorganic acid such as citric acid, lactic acid, or glycolic acid capableof chelating with the metal cation constituents, and an alcohol such asethylene glycol or glycerol capable of polymerizing the acid chelate.The result is an organic solution containing the intimately mixed andchelated metal ions. The solution is formed into a liquid film which isdried, and then heated to form a solid organic polymer or resin. Thesolid polymer or resin is fired (pyrolyzed) in oxidizing atmosphere toobtain the superconducting oxide. The film may be free-standing orsupported on a substrate (where substrate is taken to mean any form ofsupport such as ribbons, sheets, wires, tubes, coils, etc.) and has athickness that may be less than one micrometer.

In a preferred embodiment, cation compounds such as La₂ O₃, SrCO₃, CuOor Y₂ O₃, BaCO₃, CuO are dissolved in ethylene glycol and citric acid.The metals themselves (La, Sr, Cu, Y, Ba, etc.) may be directlydissolved in the ethylene glycol and citric acid. The resulting liquidis spin coated onto a flat substrate or dip coated or spray coated ontoa wire, coil, sheet, mesh or other forms, and allowed to dry. The solidpolymer is fired in an oxidizing environment leaving the superconductingoxide film. In another embodiment, a free-standing film is made bydipping a wire frame or mesh into the liquid solution and then slowlydrying the film so that a solid film is obtained. Alternatively, a filmcan be spread onto a substrate to which it does not adhere, andseparated when it is dry to produce a free-standing film.

The thin films which are synthesized using the methods of the presentinvention have unique microstructures and properties. Very thin (from afew hundred Angstroms to greater than a micrometer thickness), yet fullydense films are possible in a single coating and firing step. Filmshaving thicknesses in the range of 0.02-1.5 micrometers or greater withfully dense microstructures can be made by the methods of the presentinvention. The initial grain microstructure is very uniform and fine,with individual grain sizes of one-half micrometer or less. Withsubsequent thermal treatments, and with the use of specific crystallinesubstrates, the grain size and morphology and its orientation relativeto the substrate can be controlled over a wide range, up to andincluding substantially single crystalline films. The films are, mostimportantly, superconducting, with properties superior to the samecompositions when prepared in bulk form due to the control overmicrostructure and composition which can be exercised.

The present process of making superconducting oxide thin films hasseveral advantages over conventional high-vacuum electronic thin filmprocessing methods such as sputter-deposition, electron beam deposition,and molecular beam epitaxy. The present process is simpler, far moreeconomical, and a broader range of film compositions can be made, i.e.,many different cation constituents can be easily incorporated into thefilm. Furthermore, for a given composition the concentration of eachconstituent can be accurately controlled (to better than 1%). The filmsproduced by the process disclosed herein are extremely homogeneous andbetter control over film composition and microstructure is possible thanwith other techniques. Also, supports which are not flat such as wiresand more complex forms can be uniformly coated.

The present process is also advantageous when compared to other chemicalprocesses of preparing a ceramic thin film, such as precipitation fromsolution or sol-gel processes based on the hydrolysis of alkoxidestechniques. The present process is a closed system in that all cationconstituents in the organic liquid are incorporated into the solid film.The present process is consequently not as sensitive to solutionchemistry, since there are no precipitation steps in which the rate ofdeposition of each desired cation constituent out of liquid solution andthe physical form of the precipitant must be controlled. Homogeneity andexact control of composition are consequently better as the preferentialor incomplete precipitation of a particular constituent out of solutioncannot take place. A very broad range of cation constituents can beincorporated into the thin film as they are soluble in the organicsolution and the subsequent polymer. Formation of a thin film is alsosimpler since in the present process the organic solution wets andeasily coats a variety of support materials, and since the liquid filmis directly transformed into the solid superconducting film upon firingwithout the need for first precipitating a solid precursor from a liquidsolution onto the substrate.

The flexibility in compositions which can be synthesized by the presentprocess also allows multiple layers of different compositions to beformed. These multiple layers can be formed by re-coating with a polymersolution after each firing of the underlying layer. The sequence oflayers which can be formed is completely general, and can be forinstance layers of different superconducting compositions, oralternating layers of non-superconducting oxides and superconductingoxides. An important application is the formation of barrier or adhesionlayers of another oxide between the superconducting oxide and thesubstrate. One example involves coating the substrate using a citricacid - ethylene glycol precursor to SrTiO₃ which is fired to form adense thin film of SrTiO₃, upon which the superconducting thin film isformed. This barrier layer of SrTiO₃ inhibits possible reactions betweenthe superconducting oxide and the substrate and promotes good adhesion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a scanning transmission electron microscope photomicrograph ofa free-standing superconducting thin film of La₁.85 Sr₀.15 CuO_(4-y)with a critical temperature greater than 30K;

FIG. 2 is a scanning transmission electron microscope photomicrograph ofa dense superconducting La₁.85 Sr₀.15 CuO_(4-y) thin film of criticaltemperature greater than 30K formed on a polycrystalline aluminum oxidesubstrate;

FIG. 3 is a photomicrograph of a thicker film of the material of FIG. 2which is not fully dense but is also superconducting;

FIG. 4 is a photomicrograph of a dense superconducting YBa₂ Cu₃ O_(7-x)film of critical temperature greater than 90K formed on an amorphousSiO₂ substrate; and

FIG. 5 is a photomicrograph of the material of FIG. 4 formed on an MgOsingle crystalline substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention are described by way of thefollowing examples.

The Organic Solution

Individual stock solutions of citric acid and ethylene glycol were made,of pH 1 to 3, into which La₂ O₃, CuO, Y₂ O₃, SrCO₃, and BaCO₃ were eachseparately dissolved. Other alkoxides, hydroxides, nitrates, carbonatesand oxides of alkaline earth or rare-earth cations or transition metalcations are similarly soluble. Other forms of Cu such as Cu-nitrates, Cuhydroxide, and Cu₂ O are also soluble. The metals themselves may also bedirectly dissolved. When the metals are dissolved directly, oxygen toform the superconducting oxide films comes from the solution or from theoxidizing environment during firing. These individual stock solutionsare then mixed in the proportions necessary to obtain desired finalcompositions. Alternatively, all of the cations or cation compoundsnecessary for a desired composition can be simultaneously dissolved in asingle batch of citric acid and ethylene glycol.

EXAMPLE 1: La-Sr-Cu-O (A)

500 ml ethylene glycol

400 gm citric acid monohydrate

20 gm La₂ O₃

(B)

500 ml ethylene glycol

400 gm citric acid monohydrate

20 gm CuO

(C)

SrCO₃

Stock solutions (A) and (B) are assayed to determine the exactconcentration of La and Cu per unit volume or unit weight of thesolution, and then weighed amounts of the solutions are mixed in thedesired proportions, and SrCO₃ is added in the correct amount to obtaina final composition (after firing) of La₁.825 Sr₀.175 CuO_(4-y). In oneexample solution (A) after heating and mixing is assayed to have a Laconcentration of 1.64×10⁻⁴ moles La/ml, and (B) has 1.75×10⁻⁴ molesCu/ml, and a mixture of 145 ml of (A), 75 ml of (B), and 0.3372 g ofSrCO₃ yields a solution which can form about 5 gms of La₁.825 Sr₀.175CuO_(4-y). The mixture is heated to less than 100° C. while stirring, inorder to dissolve the SrCO₃ and mix homogeneously.

EXAMPLE II: Y-Ba-Cu-O

Y₂ O₃ is substituted for La₂ O₃ in solution (A) above. After assaying itis mixed with solution (B) in the correct measured amount, and thecorrect weight of BaCO₃ added, in order to obtain a homogeneous liquidmixture as above, which when pyrolyzed yields a composition YBa₂ Cu₃O_(7-x). In one example solution (A) is assayed to have 1.80×10⁻⁴ molesY/gm, (B) to have 1.20×10⁻⁴ moles Cu/gm, so that a mixture of 49.45 gmof solution (A), 187.57 gm of (B), and 2.960 gm of BaCO₃ yields asolution from which about 5 gms of YBa₂ Cu₃ O_(7-x) can be made.

Forming a Polymer Film

The citrate organic solution, upon heating to about 100-200° C.,evaporates solvent and becomes more viscous, and eventually becomes asolid polymer or resin. This citrate polymer film, whether deposited ona substrate or free standing, loses carbon on firing in an oxidizingatmosphere (pyrolysis) and becomes a superconducting oxide. A typicalfiring is 1/2 hour at 800° C. for the La₁.825 Sr₀.175 CuO_(4-y) and 900°C. for the YBa₂ Cu₃ O_(7-x), following which film is slowly cooled at arate of 100° C./hour or less, to room temperature, in an atmosphere ofair or oxygen.

EXAMPLE I

The film can be made in free standing form by utilizing a supportingframework to obtain a liquid film, such as by dipping a wire frame ormesh, and then slowly drying the film so that a solid polymer film isobtained. The film, when fired, yields the superconducting thin film.

EXAMPLE II

Alternatively, a film can be spread onto a substrate to which it doesnot adhere, and separated when dry.

EXAMPLE III

A supported (adherent) film is made by coating a substrate, which may beflat or of more complex shape, with the citrate liquid, drying, andfiring. Dipping of a substrate into the liquid generally does not yieldas uniform and thin a polymer film as spin coating or spray coating butcan nonetheless be used. Spin-coating, in which the substrate is spun athigh speed while the liquid is applied, yields very uniform coatings andis the preferred method of coating flat substrates at present. Theliquid can also be sprayed onto the substrate for a uniform coating.Drying of spin-coated films can be accomplished by spinning for extendedtimes in air, or by heating the substrate and film while spinning, or byheating the substrate and film after spinning. Thinner films can beprepared by spinning at higher speed while coating, or by diluting thecitrate liquid with a suitable solvent such as water or ethylene glycol.

The substrate can also be a wire or fiber, in which case a dense andcontinuous superconducting coating is formed on the wire to yield asuperconducting wire or other component. A ribbon, coil, tube or othercomplex shape can also be used as the substrate. Spin-coating is notapplicable to these forms of substrate, but dip-coating and spraycoating are.

EXAMPLE IV

A coated wire or fiber can be made by passing a continuous wire throughthe polymer solution, or by spray-coating of the wire, followed bydrying and firing. Metallic wires as well as non-metallic fibers such asglass fibers or silicon carbide fibers (Nicalon) or alumina fibers orsilicon carbide CVD filaments are suitable for coating. Also, carboncoated with silicon carbide may be used as a substrate.

Formation of the thin polymer film is facilitated by the use of asubstrate material which is wet, or partially wet, by the citrateliquid. Ceramic substrates which can be used include SiO₂, SiC, Si,SrTiO₃, BaTiO₃, MgO and polycrystalline Al₂ O₃. Metallic substrates canalso be used, including copper, iron, stainless steel, nickel, platinum,cobalt, gold, tantalum, and their alloys.

Single crystalline substrate materials can be used for the purpose ofinducing preferred crystalline orientation in the thin film and/orpreferred grain size and orientation if the film is polycrystalline. Thefilm can also be of varying porosity and greater thickness if desired.These characteristics are controlled by varying the cation content inthe organic solution, the thickness of the polymer film, firingconditions, and other processing parameters. The process can also berepeated to build up layer thickness or to fill in porosity in the film.

FIG. 1 shows a free-standing thin film of La₁.85 Sr₀.15 CuO_(4-y)composition which is very homogeneous composition, as determined byenergy-dispersive X-ray analysis in the scanning transmission electronmicroscope, as well as being uniformly thick (about 0.5 micrometers)across the film, of a uniform grain size of about 0.5 micrometers, andwhich measurements on bulk samples have shown to be superconducting. Thefilm is one grain thick; i.e. the grains penetrate through the film.FIG. 2 shows a similar La₁.85 Sr₀.15 CuO_(4-y) thin film formed on apolycrystal substrate, in which a thin (about 0.1 micrometer) continuousfilm coats the individual grains of aluminum oxide. This film is alsocompletely dense, except for where cracks have occurred between grains.FIG. 3 shows a thicker film of the same material which is not fullydense, but rather is porous, derived from a thicker polymer filmcoating. FIG. 4 shows a YBa₂ Cu₃ O_(7-x) film formed on an amorphousSiO₂ substrate, which is completely continuous and very smooth. FIG. 5shows the YBa₂ Cu₃ O_(7-x) thin film formed on an MgO single crystallinesubstrate in which a small defect in a thicker portion of the film showssome cracking, while the adjacent film region is thin, continuous, andsmooth.

EXAMPLE V

A substrate, flat or otherwise, is first coated with a citrate precursorto SrTiO₃ which is made as follows. A solution containing:

(1) 225 ml titanium tetra-isopropanol

(2) 500 ml ethylene glycol

(3) 400 gm citric acid monohydrate

is stirred and heated to less than 100° C. until the isopropanol presenthas evaporated. This solution is assayed to determine the exactconcentration of Ti per unit volume or unit weight of the solution, andthen SrCO₃ is added in the correct amount to obtain a molar Sr:Ti ratioof 1:1. This citrate solution is used to coat the substrate in one ofthe ways described above, and fired to 600° C. in air or oxygen to yielda thin dense film of SrTiO₃. Then, the polymer solution which yields thesuperconductivity oxide is used to coat the SrTiO₃ film. After firing asecond time, a layer of superconducting oxide on top of the SrTiO₃ filmis formed. A BaTiO₃ film can be substituted for the SrTiO₃ by addingBaCO₃ in place of SrCO₃ to the Ti solution, in the same molar content.

Other embodiments are included within the scope of the claims.

What is claimed is:
 1. A method of preparing a copper oxide basedsuperconducting film comprising the steps of:making an organic liquidsolution of the cation constituents of the oxide film by chelating aprecursor solution containing said cation constituents with an organicacid and adding a polymerizing alcohol; forming the solution into aliquid film; drying and heating the liquid film solution to obtain asolid organic polymer or resin; and firing the polymer to obtain thecopper oxide based superconducting oxide.
 2. The method of claim 1wherein the film is free-standing.
 3. The method of claim 1 wherein thefilm is supported on a flat compatible substrate.
 4. The method of claim1 wherein the film is supported on a ribbon, wire, coil or otherthree-dimensional compatible substrate.
 5. The method of claim 1 whereinthe film thickness is less than one micrometer.
 6. The method of claim 1wherein the cation constituents of the oxide film, the thickness of thefilm, and the firing conditions are selected so that the film is dense.7. The method of claim 1 wherein the cation constituents of the oxidefilm, the thickness of the film, and the firing conditions are selectedso that the film is porous.
 8. The method of claim 1 where multiplecoatings are used to form thicker or more dense solid superconductingfilm.
 9. The method of claim 2 wherein a supporting framework supportsthe superconducting film.
 10. The method of claim 9 wherein thesupporting framework is a wire frame or mesh.
 11. The method of claim 10wherein the wire frame or mesh is dipped into the organic liquidsolution to form the film.
 12. The method of claim 2 wherein the liquidfilm is spread on a substrate, dried, and separated from the substrate.13. The method of claim 3 or 4 wherein the substrate is a singlecrystalline material.
 14. The method of claim 3 or 4 wherein thesubstrate is dipped into the organic liquid solution to form a liquidfilm.
 15. The method of claim 3 wherein the organic liquid solution isapplied to a spinning substrate.
 16. The method of claim 3 or 4 whereinthe substrate is wet or partially wet by the organic liquid solution.17. The method of claim 3, 4, 13, 14, 15, or 16 wherein the substrate ischosen from the group consisting of SiO₂, SiC, Si, SrTiO₃, BaTiO₃, MgO,carbon coated with SiC, Al₂ O₃, stainless steel, copper alloys, nickelalloys, cobalt alloys, gold alloys, platinum alloys and tantalum alloys.18. The method of claim 15 wherein the film is dried by spinning in air.19. The method of claim 15 wherein the film is dried by heating thesubstrate and film while spinning.
 20. The method of claim 15 whereinthe film is dried by heating the substrate and film after spinning. 21.The method of claim 1 wherein the cations are chosen from the groupconsisting of La, Ba, Cu, Y, Sr, Ca, and rare-earth ions such as Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu.
 22. The method of claim 1 wherein the oxidefilm comprises La_(2-x) M_(x) CuO_(4-y) where M is an alkaline earthcation.
 23. The method of claim 1 wherein the oxide film comprises NBa₂Cu₃ O_(7-x) where N is Y, or a rare-earth ion such as La, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu.
 24. The method of claim 1 wherein the organicliquid solution includes ethylene glycol as the polymerizing alcohol andcritic acid as the chelating organic acid.
 25. The method of claim 1wherein the cation constituents are introduced as compounds chosen fromthe group consisting of La₂ O₃, CuO, Y₂ O₃, SrCO₃, BaCO₃, and Cu₂ O. 26.The method of claim 20 wherein the liquid film solution is dried at atemperature in the range of 100°-200° C.
 27. The method of claim 1wherein the solid organic polymer is fired in an oxidizing atmosphere.28. A method of claim 1 wherein the cations comprise La, Sr, and Cu. 29.The method of claim 1 wherein the oxide film comprises La₁.825 Sr₀.175CuO_(4-x) where x is less than 1.0.
 30. The method of claim 1 whereinthe cations comprise Y, Ba and Cu.
 31. The method of claim 1 wherein theoxide film comprises YBa₂ Cu₃ O_(7-x) where x is less than 1.0.
 32. Themethod of claim 1 wherein the organic acid is citric acid.
 33. Themethod of claim 1 wherein the organic acid is lactic acid.
 34. Themethod of claim 1 wherein the organic acid is glycolic acid.
 35. Themethod of claim 1 wherein the alcohol is ethylene glycol.
 36. The methodof claim 1 wherein the alcohol is glycerol.
 37. The method of claim 28wherein the polymer is fired for approximately one-half hour atapproximately 800° C. and then cooled to room temperature.
 38. Themethod of claim 30 wherein the polymer is fired for approximatelyone-half hour at approximately 900° C. and then cooled to roomtemperature.
 39. The method of claim 3 wherein the substrate is a singlecrystalline material for inducing a preferred crystalline orientation inthe thin film.
 40. Method of preparing a multiple layer copper oxidebased superconducting oxide film comprising the steps of:making anorganic liquid solution of the cation constituents of a firstsuperconducting oxide film layer by chelating a precursor solutioncontaining said cation constituents with an organic acid and adding apolymerizing alcohol; forming the solution into a liquid film; dryingand heating the liquid film solution to obtain a solid organic polymeror resin; firing the polymer to obtain a first layer of thesuperconducting oxide; making an organic liquid solution of the cationconstituents of a second superconducting oxide film layer by chelating aprecursor solution containing the cation constituents of the secondsuperconducting oxide film with an organic acid and adding apolymerizing alcohol; forming the solution into a liquid film on thefirst superconducting oxide layer; drying and heating the liquid filmsolution to obtain a second solid organic polymer or resin; firing thesecond polymer to obtain a second superconducting oxide layer; andrepeating the process for additional superconducting oxide layers.